Photolithographic masks—frequently also referred to in general as photomasks, masks or reticles—are projection templates, the most important application of which is photolithography for producing semiconductor components, in particular integrated circuits. As a result of the continuously growing integration density in the semiconductor industry, photolithographic masks must image ever smaller structures onto a light-sensitive layer, i.e. onto a photoresist on wafers. To meet this requirement, the exposure wavelength of photolithographic masks has been shifted from the near ultraviolet via the mid-ultraviolet into the far ultraviolet range of the electromagnetic spectrum. Currently, use is normally made of a wavelength of 193 nm for exposing the photoresist on wafers. Future lithography systems will probably operate with wavelengths in the extreme ultraviolet (EUV) range (preferably but not necessarily in the range of 6 nm to 15 nm). The currently particularly preferred wavelength is 13.5 nm.
The EUV wavelength range places huge demands on the precision of optical elements in the beam path of future lithography systems. In all probability, the optical elements, and hence also the photolithographic masks, will be reflective optical elements. Said masks have a multilayer structure, e.g. of a periodic sequence of molybdenum and silicon. Additionally, an absorber structure of absorbing pattern elements is applied to the multilayer structure. In the regions of the EUV mask covered by pattern elements of the absorber structure, incident EUV photons are absorbed or at least not reflected like in other regions. As a consequence, the production of photolithographic masks with increasing resolution becomes ever more complex and therefore also ever more expensive.
Photolithographic masks must be largely error-free, since an error of the mask would reproduce on each wafer during each exposure. In the case of a photolithographic mask, it is important that the pattern elements of the absorber structure on the photolithographic mask exactly image the structure elements predetermined by the design of the semiconductor element into the photoresist on the wafer. The intended dimension of the structure elements produced in the photoresist by the absorber pattern is referred to as the critical dimension (CD). This dimension and the variation thereof (CDU, critical dimension uniformity) are central characteristic variables for the quality of a photolithographic mask. Freedom of errors for photolithographic masks in this context means that the mask upon exposure with the actinic wavelength images an intended dimension within a predetermined error interval onto a wafer, i.e. the CD may only vary within the predetermined error interval. If this condition is satisfied, the photolithographic mask has no visible defects or printable defects on a wafer. Since not every defect is a printable effect, masks are examined using mask metrology apparatuses. By way of example, AIMS and the series WLCD by Carl Zeiss SMT GmbH are mentioned.
It is known that in the transition from UV wavelengths, such as e.g. 193 nm, to EUV wavelengths, statistical effects of photons play a role. The wavelength reduction from 193 nm to 13.5 nm in lithography apparatuses results in a photon energy which is approximately 14 times higher. The reduction in size of the lithographically defined structures furthermore results in a surface density of the structures which is approximately 5 to 10 times higher. As a consequence, approximately two orders of magnitude fewer photons per structure element are used in EUV lithography, with the result that the statistical effects increase considerably. These effects are described for example in “Impact of Stochastic effects on EUV printability limits,” P. De Bisschopa et al., 2014, Proc. of SPIE Vol 9048 and “Contribution of EUV mask CD variability on LCDU,” Zhengqing John Qi et al., 2017, Proc. of SPIE 10243.
The statistical effects are caused firstly in the aerial image due to interaction of the photons with the photomask, and in addition, statistical effects likewise occur during the exposure in the photoresist. Typical characteristic variables on the exposed wafer are, e.g. the line edge roughness (LER), line width roughness (LWR) and the local variation in the wafer-side critical dimension (LCDU, local critical dimension uniformity). In particular for the masks and the process control and for the development of the qualification of lithographic processes and photoresists, a separation of the contributions by aerial image and photoresist with respect to the statistical processes is advantageous.